Field of the Invention
The invention relates to the manufacture of containers, such as bottles or jars, obtained by stretch blow molding of preforms made of thermoplastic material, such as polyethylene terephthalate (PET), high-density polyethylene (HDPE), or any other known material.
The invention relates more particularly to a method for manufacturing containers made of plastic material, such as PET, by stretch blow molding of preforms, in a mold, with a pressurized fluid, particularly air, of the type in which the machine for its implementation comprises a control system, a thermal conditioning oven, and a blow-molding unit comprising at least one blow-molding station, said station having a mold, into which the preforms are introduced, coming from the oven, each one to undergo an operation of transforming into a container, the operation including a stretching stage (also called an elongating stage) of the preform, using an elongating rod that is associated with the mold and is controlled in sliding toward the bottom of the mold.
Description of the Related Art
The stretching and the blow molding of the body of a preform require that it be brought to a temperature that is higher than the glass transition temperature of the material. Thus, first of all, a thermal conditioning of the preform is initiated by making it circulate inside an oven. The oven comprises heating means that are, for example, formed from infrared lamps. The preform is moved into the oven by a conveying system.
Then, the heated preform is introduced into the mold, and then it is stretched by means of a sliding rod (called stretching or elongating rod), and pressurized gas is introduced into the preform to transform it into a container by blow molding. The introduction of the pressurized gas in all cases comprises a blow-molding stage itself, which consists in introducing into the preform a gas, generally air under high pressure (typically between 18 and 40 bars). The blow-molding stage ordinarily is preceded by a first stage, called pre-blow molding, which consists in introducing a gas having a lower pressure (between 8 and 15 bars) while the elongating rod, having reached the bottom of the preform, causes its longitudinal stretching. The stretching, pre-blow-molding and blow-molding (alternatively stretching and blow-molding) stages occur according to a pre-established sequence during the parameterizing of the machine, a sequence that takes into account the preforms used and the shape of the container to be obtained. The stretching and the blow molding (or the pre-blow molding and the blow molding) make it possible for the material that makes up the preform to undergo a molecular double orientation, which imparts to the final container particular mechanical properties. The pre-blow molding starts the deformation of the preform to transform it into a container, and the blow molding makes possible an optimal taking of the impression in the mold, so that the details of the container are well marked.
In the following description, unless other details are introduced, the expression “blow-molding process” will be used equally to designate a sequence comprising a pre-blow-molding stage followed by a blow-molding stage or to designate a process that comprises only a blow-molding stage. Consequently, “to begin the blow-molding process” will signify either to start the pre-blow-molding stage by injecting the pre-blow-molding gas in the case where such a stage exists, or to start the blow-molding stage directly by injecting the blow-molding gas.
After a certain time of contact of the plastic material against the mold, during a degassing stage, the pressure in the container is brought back to the atmospheric pressure before removing the final container from the mold. In other methods, the degassing stage is preceded by a stage for recycling a portion of the fluid contained in the container, so as to reinject it toward other uses (in the machine itself or in the factory where the machine is installed). There can also be a stage known as flushing for the bottles that are hot-fillable, during which a circulation of air ensures the cooling of the container that is in contact with a hot mold.
The preforms are generally obtained by injection of the material into dedicated injection molds. They have a tubular cylindrical body that is closed at one of its axial ends, which is extended at its other end by a neck, it also being tubular. The neck of the preform is generally injected in such a way as to already have the shape of the neck of the final container, while the body of the preform is called on to undergo a relatively significant deformation to form the body of the final container, following the blow-molding operations. The necks of the preforms often have bearing collars intended to hold them on the upper edge of the molds, during the formation of the containers.
A container has a side wall (also termed body), a neck that extends from an upper end of the body, and a bottom that extends from a lower end of the body, opposite the neck. The bottom of the container defines a seat, generally at the junction with the body, and by which the container can rest on a flat surface (such as a table).
The mold comprises a wall defining a cavity intended to impart its shape to the body of the container. This cavity is closed, at a lower end, by a mold bottom intended to impart its shape to the bottom of the container.
Today, to parameterize a machine to produce a given type of container, there is placed in a mold of the machine, while it is stopped, a preform emerging from injection (therefore cool and not having undergone reheating) corresponding to those that will be used for this type of container, and then the position of the elongating rod when it reaches the bottom of the preform is determined. The position at which the end of the elongating rod reaches the bottom of the preform is conventionally called by the applicant point zero (“Point 0”) of the elongation. From Point 0, the continuation of the movement of the elongating rod causes the elongation of the preform. During the parameterizing, a position of the elongating rod is also established at which the blow-molding process must begin, which can correspond to Point 0 or be located beyond this point. In other words, a theoretical stretching length between Point 0 and the beginning of the blow-molding process is determined. Actually, as soon as the preform begins to be stretched, if nothing else is done, the material making up the preform is tightened on the rod. A first consequence is a risk of excessive cooling of the zones of the preform in contact with the elongating rod (the elongating rod is generally cool) leading to a container of very poor quality, because it then becomes impossible to deform these zones well because of their cooling. Another consequence is a risk of damage, particularly by piercing, of the bottom of the preform by the elongating rod.
Other operations are also parameterized between the time when the elongating rod reaches the bottom of the preform and the time when the bottom of the preform, driven by the rod, reaches the bottom of the mold (called Point 10). Other operations still take place after reaching Point 10.
This way of doing things has various drawbacks.
Actually, the determination of Point 0 is made with the machine stopped, on a cool preform that has emerged from the injection press. Now, it has been found that apparently identical preforms could have a different behavior after their passage into the thermal conditioning oven of the blow-molding machines.
In particular, it has been found that after heating in the oven, the length of some preforms could vary, the difference being able to reach 5 to 10 mm from one preform to the next. In particular, it has been found that for preforms of slight thickness, used to produce small-sized bottles, for example bottles of 0.5 liter, and with slight wall thicknesses, the length of such preforms at the oven outlet could be more than 10 mm shorter in relation to their length at the entrance to the oven, in other words in relation to their length at the outlet of the injection press. In the case of preforms intended to be transformed into small-sized bottles (typically lightweight water bottles of 0.5 liter), such a reduction in length, also called retraction or “shrinkage” by a person skilled in the art, corresponds to a longitudinal variation of about 15%. This reduction is also accompanied by an increase in the diameter of the preform. For the same type of bottles, increases in diameter of about 7% have been noted. On the other hand, for the same production of preforms, differences of several mm between the maximum retraction and the minimum retraction can also exist.
The existence of a retraction during the heating of the preforms results in the generating of stresses in the preforms during their manufacture. These stresses appear because of the pressures or else of the temperature to which the material is subjected during the injection. During the heating, a portion of the stresses is relaxed with, as a consequence, a reduction in the length and an increase in the diameter of the preforms. The relaxing of the stresses is sometimes called “relaxation.”
Differences in retraction have been noted not only for the same production of preforms or on preforms of the same type, but even from one type of preform to the next. These phenomena are made worse by the current tendency to reduce the weight of the containers and therefore that of the preforms and of their thickness.
For the same type of preforms, the various retraction phenomena and the differences can be explained in the following way: when the preforms are obtained using the same press that can contain several tens of cavities (presses with one hundred cavities are known), it can happen that, from one cavity to the next, there is not exactly the same quantity of material that is injected, or else that one cavity is less well cooled than the next. Furthermore, a single machine for manufacturing containers must be supplied with several hundreds of thousands of preforms (some going beyond one million) per day of production, and it is conceivable that the preforms intended for such a machine do not exit from the same injection press. In this case, variations from one press to the next can occur, because of possible differences of adjustment of the parameters between two presses. Finally, other parameters can have an influence, such as the intrinsic quality of the injected material.
The retraction differences from one type of preform to the next can have one or more origins, among which in particular are the different parameters of the designs of the preforms, the thicknesses of the preforms, the specifications of the resins and/or the parameters of the injection processes, particularly the injection pressure, even though the weights of the preforms would be identical.
As a consequence, the containers produced will be able to have different distributions of material, given that some preforms will not have undergone retraction, while others will have undergone it that, moreover, will be able to be different from one preform to the next.
Furthermore, inasmuch as some preforms will have become shorter than others before their introduction into the mold, the actual stretching will begin before reaching Point 0 determined during the parameterizing. Now, the stretched length of the preform before the beginning of pre-blow molding has considerable influence on the distribution of the material of the container (for certain containers, the stretching must be zero). Also, if the stretching begins too early in relation to the pre-blow molding, or if a stretching takes place when it should not have, and if the retractions are different from one preform to the next, different distributions of material will appear on the containers, with a risk of tightening of the material on the rod, a risk that is all the higher as the containers are manufactured with a high rate of elongation.
The current machines are increasingly automated, which makes it possible to correct various time-based drifts of the blow-molding method. It is known (see the document WO2008/081107 in the name of the applicant) to correlate unique points of an actual curve of blow molding with machine parameters (particularly the flow rate or the pressure of pre-blow molding), and to apply corrections of the parameters as a function of divergences found at these unique points. However, automation does not make it possible to correct defects such as those mentioned above. Actually, a drift or drifts caused by the modification of the physical characteristics of a preform cannot be corrected by the adjustment method described in the previously-cited document WO2008/081107.